<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>


        /* width 子元素不设置宽度，会自动继承父元素的宽度 */


    .parent
    {
        width: 200px;
        height: 200px;
        background-color: red;
    }
    .child{

        height: 50px;
        background: blue;
    }
    
    
    </style>
</head>
<body>
    <div class="parent">
         <div class="child">

         </div>
    </div>
</body>
</html>